In a turbocharger engine, a turbocharger configured to supercharge intake air by using exhaust energy of an engine is mounted adjacent to a side wall of an engine body. An exhaust passage and an intake passage are formed in a housing of the turbocharger. A turbine chamber configured to accommodate a turbine communicates with the exhaust passage. A compressor chamber configured to accommodate a compressor impeller communicates with the intake passage. Exhaust air is supplied to the exhaust passage from the engine body, and intake air to be supplied to the engine body flows through the intake passage. The turbine is rotated around a turbine shaft by exhaust air to rotate the compressor impeller in the compressor chamber, which is connected to the turbine shaft to supercharge intake air.
Conventionally, there is a known turbocharger in which two independent turbo units are disposed in series in an exhaust passage. For instance, U.S. Pat. No. 8,397,503 discloses a dual stage turbocharger provided with a large turbo unit configured to be mainly operated in an intermediate speed range to a high speed range of an engine, and a small turbo unit configured to be mainly operated in a low speed range of the engine. Each of the large turbo unit and the small turbo unit includes a turbine chamber, a compressor chamber, and a turbine shaft extending between the turbine chamber and the compressor chamber.
In a turbocharger, there is a demand for supplying intake air to a compressor chamber while reducing a resistance (an intake resistance) against a flow of intake air as much as possible in order to improve supercharging efficiency. In the case of an engine provided with a dual stage turbocharger, however, it is necessary to miniaturize the turbocharger regardless of complication of the intake passage layout. Therefore, it may be difficult to reduce an intake resistance. Further, usually, a turbocharger is mounted on an engine body in such a manner that a turbine shaft of the turbocharger is aligned in parallel to a crankshaft (an output shaft) of the engine body. The inventors of the present application found that the aforementioned matter is one of the reasons why the flexibility of the intake passage layout is restricted.